Static correction for magnetotelluric data

ABSTRACT

A method for removing the effects of said subsurface anomalies in magnetotelluric survey data is disclosed wherein resistivity curves are compared and a correction factor is determined. Resistivity curves having a deviation from a mean are multiplied by this correction factor.

BACKGROUND OF THE INVENTION

The present invention pertains to a method for processingmagnetotelluric survey data and more particularly to a method forremoving the effects of near surface anomalies on resistivity curvesproduced through magnetotelluric surveys.

Several tools for geological exploration research of the earth'ssubsurface formations exist in present technology. Although the mostwidely accepted tool is seismic surveying, other tools such asmagnetotelluric surveys may be used in certain instances. Themagnetotelluric survey is usually less expensive than a seismic survey.However, in general, a magnetotelluric survey is not as accurate nordoes it possess as high a degree of resolution as a seismic survey.

A method for taking magnetotelluric measurements is to place fourelectrodes in a pattern defining a square with each electrode in acorner. Of the four electrodes, opposite corners of the square arepaired, thus the lines defined by connecting paired electrodes areperpendicular to each other. Electrical impulses are received fromnatural electrical energy in the earth from one electrode to the otherelectrode of the pair. For complete magnetotelluric survey informationmagnetometer coils are needed to measure the magnetic effects of theearth at the location being surveyed. The electrode pairs give the Ecomponents of the survey, normally designated as E_(x) and E_(y). The Emeasurements are correlated with H measurements (H_(x) and H_(y)) whichare detected by the magnetometer coils. The frequency of the electricalwaves indicates the depth of the formation for which the resistance ismeasured. For example, the resistance of a shallow subsurface formationcan be measured by detecting high frequency telluric electromagneticwaves. To obtain the resistance of deeper formations, lower frequenciesof the telluric electromagnetic waves are measured. The depth of theformation having the measured resistivity is calculated by combining thetelluric electrical field and the magnetic field measured. Resolution ofthe exact point for which the resistivity is being measured at a givendepth deteriorates as a greater depth is surveyed. For a more detaileddiscussion of magnetotelluric surveying and electrode placementtechniques, reference is made to U.S. Pat. No. 4,286,218 titled"Multiple Site Magnetotelluric Measurements" Ser. No. 063,491 issued toMarvin G. Bloomquist, et al, assigned to the same assignee as thepresent application.

In a magnetotelluric survey, the resistivity of the subsurfaceformations beneath the measuring device is frequently the informationdisplayed. This information may be plotted illustrating resistivity as afunction of frequency such as the graphical representation of FIG. 1. Asillustrated in FIG. 1A, by convention two separate resistivity curves(R_(x) and R_(y)) are routinely generated during the processing ofmagnetotelluric data. These two curves are assumed to be "parallel toelectrical strike" and "perpendicular to electrical strike"respectively. From the two curves labelled R_(x) and R_(y), approximatetrue resistivity versus depth curves are calculated in an attempt togenerate a highly smooth resistivity distribution of the subsurface.However, due to surface or near surface anomalies, the R_(x) and R_(y)curves will separate or split into R'_(x) and R'_(y) curves and beparallel to each other. Thus a DC like bias is present in the data andillustrated in FIG. 1B. This DC bias is normally attributed to localizedsurface anomalies such as "pipelines, geological faults, lithologyvariations, small caverns", etc. A subsurface formation which dips willprduce different readings depending on the relationship between theconfiguration of the measuring electrode pair and the angle of theformation dip.

SUMMARY OF THE INVENTION

The present invention removes the effects of undesirable subsurfaceanomalies present in magnetotelluric data by establishing a correctionfactor to normalize the resistivity curves obtained from each site. Aportion of each resistivity curve is averaged to establish mean valueand variance limit. All resistivity curves which are outside thevariance limits are multiplied by a ratio established by the mean valueand the portion averaged for that resistivity curve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are graphical representations of R_(x), R_(y) andR'_(x), R'_(y) components of resistivity curves.

FIG. 2 is a graphical representation of a resistivity curve.

FIG. 3 is a vertical cut-away representation of a section of the earth'ssubsurface.

FIGS. 4A, 4B, are graphical representations of resistivity curves.

FIG. 5 is a graphical representation of portion averages for typicalresistivity curves.

FIG. 6 is a flow chart of the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As previously stated in conjunction with FIGS. 1A and 1B, resistivitycurves R'_(x) (resistivity parallel to strike) and R'_(y) (resistivityperpendicular to strike) of FIG. 1B may split due to the effects ofsubsurface anomalies which are unrelated to natural formations, such aspipelines, or small natural formations, such as small caverns, biasingthe entire resistivity of the curve. Curves R_(x) and R_(y) of FIG. 1Amay partially track and partially split due to deep subsurface anomaliessuch as formation dips, etc.

Referring now to FIG. 2, a graphical representation of a resistivitycurve R_(t), drawn through a series of data points, having a frequencyas the abscissa and resistance as the ordinate is illustrated. R_(t) isa resistivity curve typical of those obtained from a combination ofR_(x) and R_(y). At the higher frequencies of approximately 1 to 100hertz, the resistivity curve is almost constant. The frequency rangefrom approximately 0.01 hertz to 0.1 hertz, a subsurface formation withincreased resistivity exists. Below a frequency of aproximately 0.001hertz the resistivity of the formation increases steadily to a value ofapproximately 70 ohms. Resistivity curve R_(t) is similar to thoseobtained for each site in a magnetotelluric survey with the frequencyaxis having a relationship to a more meaningful parameter such as depth.

Referring now to FIG. 3, a cross section of a typical vertical sectionof earth is illustrated having 3 levels of resistance, R₁ as theresistance closest to the surface followed by R₂ and the deepestformation R₃. Magnetotelluric sites L₁ through L₈ are indicated acrossthe surface. Beneath site L₃ and site L₅, resistive anomalies R_(a) andR_(b) exist. For the purposes of example, we will assume that R_(a) is ahighly resistive anomaly and R_(b) is a less resistive or moreconductive anomaly.

FIGS. 4A and 4B represent the resistivity curves for the R'_(x)component of the field data. A similar, although not identical, set offigures can be drawn for the R'_(y) component.

FIG. 4A illustrates a resistivity curve such as that which would beencountered at site L₃. The increased resistivity of R_(a) has displacedthe total resistivity or true resistivity of the subsurface formationsby a small increment Δx.

FIG. 4B illustrates a resistivity curve typical of what would beencountered at site L₅. The resistivity curve is displaced downward orin a less conductive direction by a small increment Δx'. In both FIG. 4Aand FIG. 4B the normal or expected average resistance is indicated bydashed line R_(ave).

Referring now to FIG. 5, the graphical representation of the averagevalues of resistance for a portion of the resistivity curves for sitesL₁ through L₈ is illustrated. Line R_(ave) represents the meansresistance for sites L₁ through L₈. The average values for sites L₃ andL₅ have been displaced from R_(ave) by the resistance anomalies R_(a)and R_(b) immediately below the site location. For purposes of example,line R_(ave) is shown as a straight line with a resistivity value of 10ohm-meters at each site L₁ through L₈. Line R_(ave), under variousgeologic conditions, may be a curved line.

In actual practice, the distance between sites on a magnetotelluricsurvey can vary anywhere from hundreds of feet to several miles. Basedon the information from neighboring sites, discrepencies in averagevalues for resistivity curves taken at each site may be determined.However, mere discrepency alone does not indicate that the recorded datahas been effected by some subsurface anomaly but may indicate a regionhaving a resistance inconsistent with expected data. To determinewhether a subsurface anomaly is present, the individual curves whichcomprise the true or total resistivity of a site are compared. If R_(x)is displaced from R_(y) through their entire distance (see FIG. 1A) anear surface anomaly is indicated. As indicated previously, prior artteaches no solution to the erroneous data.

Referring now to FIG. 6, a flow chart of the method of the presentinvention is illustrated. Data is received by receiver 12 which may beof any type receiver presently known in the art. Receiver 12 feeds theinformation to averager 14 which selects portions of the resistivitycurves for each site and computes an average value for each of theresistivity curves. The calculated averages from averager 14 are fed toestablisher 16 and determiner 18. Establisher 16 established a meanvalue for the averages of all resistivity curves. Establisher 16 alsopasses the portion averages from averager 14 to selector 20 asillustrated in FIG. 6. Determiner 18 determines the permissible variancelimits of the resistivity curve averages. Both establisher 16 anddeterminer 18 may be programmable read only memories (PROM) such asthose presently available in the art. Both establisher 16 and determiner18 feed the information to selector 20 which determines the resistivitycurves that are to be normalized. Selector 20 compares the individualresistivity curves with the mean value and selects those which areoutside of the established variance. Selector 20 feeds this informationto ratio computer 22 which computes a ratio of the mean value and theportion averaged for each of the selected curves. This ratio along withthe resistivity curves is fed from computer 22, as illustrated in FIG. 6to multiplier 24 which normalizes the selected curves by multiplyingevery point on each curve by the compound ratio for that particularresistivity curve. Each of the 8 resistivity curves may then bedisplayed by display 26 which may be of any type currently used in theart such as cathode ray tube, printer, etc.

Averager 14 selects only a portion of each resistivity curve fed throughreceiver 12 to establish a base for the ratio calculated by computer 22.Only a portion of the curve is required since if subsurface anomalybiasing is present, a single point gives the extent of the biasing.Several points or a small portion of the curve is averaged to lessen theeffect of noise which may be present in any one point.

Establisher 16 determines a mean value for the portion averager fromaverager 14. A mean value is preferred, although a second average of theportion averages may be computed. An average of the portion averages isideally the same as a mean value of the portion averages assuming thatfor any given magnetotelluric survey the distribution between lessconductive and more conductive effects is equal. A greater number ofconductive anomalies, or a greater number of resistive anomalies willcause a significant difference between the value of the average of theportion averages in comparison to the mean average. Determiner 18establishes variance limits which determine the resistivity curves to beselected. The variance limit may be defined as a percentage differencerelative to the mean resistivity determined by establisher 16, or anyother predetermined difference common in the art.

Selector 20 receives a mean average from establisher 16 and the variancelimits from determiner 18 and also receives the portion averages of theresistivity curves from averager 14. Selector 20 compares each of theportion averages from averager 14 with the mean average from establisher16. Any of the resistivity curves which have a portion average exceedingthe variance limits established by determiner 18 are fed to computer 22and multiplier 24. Computer 22 takes the mean average value andmultiplies it by the portion average calculated for each resistivitycurve by averager 14. Thus, for example, for the resistivity curveillustrated in FIG. 4A, if the portion which was averaged is between 1and 10 cycles, a value of approximately 30 ohms is obtained. Meanaverage as indicated by dashed line R_(ave) is 10 ohms. The curve ofFIG. 4A would be multiplied by a ratio of 10/30 which would place thecurve down to a value as that illustrated by R_(t) in FIG. 2. The curveof FIG. 4B having a resistivity value of 3 ohms or the average between 1and 10 cycles would be multiplied by a ratio of 10/3 which would moveFIG. 5 also to a position as that of curve R_(t) of FIG. 2. Therefore, aratio is calculated by computer 22 to adjust each resistivity curve toproduce a consistent set of resistivity curves having the effects ofsubsurface anomalies removed. Selector 20, which may be a programmableread only memory, may be programmed to not select certain curves.Resistivity curves which have resistivity average outside the variancelimits due to unrealistic changes in formation resistance and notsubsurface anomalies are not to be normalized for the calculation ofR_(ave). These curves may be preselected to bypass computer 22. Thedetermination of which curves are to be normalized may be done inaccordance with the procedure described in conjunction with FIGS. 1A and1B. Multiplier 24 receives the ratio from computer 22 and also receivesresistivity curves selected by selector 20. Each of the resistivitycurves from selector 20 is multiplied by its corresponding ratio fromcomputer 22. Multiplier 24 multiplies each point of data on theresistivity curves received from selector 20 by the ratio computed bycomputer 22. This information is then fed to display 26 which, aspreviously pointed out, may be of any type of display presently used inthe art.

The foregoing description of the preferred embodiment has been given byway of flow chart and indications as to the type of components which maybe used for each step. However, the method of the present inventionoperates particularly well on a presently used digital computer and hasbeen intended for such use.

The present invention teaches a method which may be used to remove theeffects of subsurface anomalies, such as, pipelines, railroad tracks,small caverns, etc. Through the use of the present invention, moreaccurate information may be obtained from the use of magnetotelluricsurveys to increase their reliability and acceptance.

The present invention has been described by way of a preferredembodiment for illustration only and should not be limited thereto butonly by the scope of the following claims.

What is claimed is:
 1. A method for correcting near surface resistiveanomalies in magnetotelluric surveys comprising the steps of:receivingmagnetotelluric survey data curves; selecting a portion of each curve;computing an average for said portion of each curve; determining a meanvalue and variance limit for said averages; selecting all curves havingsaid average portion exceeding said variance limit; multiplying saidselected curves by a ratio of said average of each curve and said meanvalue to correct said selected curves; and displaying said correctedcurves and said curves having said average portion within said variancelimit.
 2. Apparatus for correcting subsurface resistive anomalies inmagnetotelluric surveys comprising:means for receiving magnetotelluricsurvey data for each of a plurality of sites; means for computing anaverage for preselected depth points for each of said sites; means fordetermining a mean value and variance limits for said averages of saidsites; means for producing a ratio of said mean value and said averagefor each site; means for multiplying the value of every data point fromeach site by said ratio to produce corrected magnetotelluric data; andmeans for displaying said corrected magnetotelluric data.